Cellular base stations typically include, among other things, a radio, a baseband unit, and one or more antennas. The radio receives digital information and control signals from the baseband unit and modulates this information into a radio frequency (“RF”) signal that is then transmitted through the antennas. The radio also receives RF signals from the antenna and demodulates these signals and supplies them to the baseband unit. The baseband unit processes demodulated signals received from the radio into a format suitable for transmission over a backhaul communications system. The baseband unit also processes signals received from the backhaul communications system and supplies the processed signals to the radio. A power supply is provided that generates suitable direct current (“DC”) power signals for powering the baseband unit and the radio. The radio is often powered by a (nominal) −48 Volt DC power supply.
In order to increase coverage and signal quality, the antennas in many cellular base stations are located at the top of a tower, which may be, for example, about fifty to two hundred feet tall. In early cellular systems, the power supply, baseband unit and radio were all located in an equipment enclosure at the bottom of the tower to provide easy access for maintenance, repair and/or later upgrades to the equipment. Coaxial cable(s) were routed from the equipment enclosure to the top of the tower that carried signal transmissions between the radio and the antennas. However, in recent years, a shift has occurred and the radio is now more typically located at the top of the antenna tower and referred to as a remote radio head (“RRH”). Using remote radio heads may significantly improve the quality of the cellular data signals that are transmitted and received by the cellular base station, as the use of remote radio heads may reduce signal transmission losses and noise. In particular, as the coaxial cable runs up the tower may be 100-200 feet or more, the signal loss that occurs in transmitting signals at cellular frequencies (e.g., 1.8 GHz, 3.0 GHz, etc.) over the coaxial cable may be significant. Because of this loss in signal power, the signal-to-noise ratio of the RF signals may be degraded in systems that locate the radio at the bottom of the tower as compared to cellular base stations where remote radio heads are located at the top of the tower next to the antennas (note that signal losses in the cabling connection between the baseband unit at the bottom of the tower and the remote radio head at the top of the tower may be much smaller, as these signals are transmitted at baseband frequencies or as optical signals on a fiber optic cable and then converted to RF frequencies at the top of the tower).
FIG. 1 schematically illustrates a conventional cellular base station 10 in which the radios are implemented as remote radio heads. As shown in FIG. 1, the cellular base station 10 includes an equipment enclosure 20 and a tower 30. The equipment enclosure 20 is typically located at the base of the tower 30, and a baseband unit 22 and a power supply 26 are located within the equipment enclosure 20. The baseband unit 22 may be in communication with a backhaul communications system 28. A plurality of remote radio heads 24 and a plurality of antennas 32 (e.g., three sectorized antennas 32) are located at the top of the tower 30. While the use of tower-mounted remote radio heads 24 may improve signal quality, it also requires that DC power be delivered to the top of the tower 30 to power the remote radio heads 24.
A fiber optic cable 38 connects the baseband unit 22 to the remote radio heads 24, as fiber optic links may provide greater bandwidth and lower loss transmissions. A power cable 36 is also provided for delivering the DC power signal up the tower 30 to the remote radio heads 24. The power cable 36 may include a first insulated power supply conductor and a second insulated return conductor. The fiber optic cable 38 and the power cable 36 may be provided together in a hybrid power/fiber optic cable 40 (such hybrid cables that carry power and data signals up an antenna tower are commonly referred to as “trunk” cables). The trunk cable 40 may include a plurality of individual power cables that each power a respective one of the remote radio heads 24 at the top of the antenna tower 30. The trunk cable 40 may include a breakout enclosure 42 at one end thereof (the end at the top of the tower 30). Individual optical fibers from the fiber optic cable 38 and individual conductors of the power cable 36 are separated out in the breakout enclosure 42 and connected to the remote radio heads 24 via respective breakout cords 44 (which may or may not be integral with the trunk cable 40) that run between the remote radio heads 24 and the breakout enclosure 42. Stand-alone breakout cords 44 are typically referred to as “jumper cables” or “jumpers.” Coaxial cables 46 are used to connect each remote radio head 24 to a respective one of the antennas 32.
The DC voltage of a power signal that is supplied to a remote radio head 24 from the power supply 26 over a power cable 36 and breakout cord 44 may be determined as follows:VRRH=VPS−VDrop  (1)where VRRH is the DC voltage of the power signal that is delivered to the remote radio head 24, VPS is the DC voltage of the power signal that is output by the power supply 26, and VDrop is the decrease in the DC voltage that occurs as the DC power signal traverses the power cable 36 and breakout cord 44 that connect the power supply 26 to the remote radio head 24. VDrop may be determined according to Ohm's Law as follows:VDrop=IRRH*RCable  (2)where Rcable is the cumulative electrical resistance (in Ohms) along the power supply and the return conductors of the power cable 36 and breakout cord 44 that connect the power supply 26 to the remote radio head 24, and IRRH is the average current (in Amperes) flowing through the power cable 36 and breakout cord 44 to the remote radio head 24. As is readily apparent from Equation 2, as the current IRRH drawn by a remote radio head 24 increases, the voltage drop VDrop along the power cable 36 will increase as well. The voltage drop VDrop of Equation 2 is also referred to herein as the I*R voltage drop.
The power cables 36 and breakout cords 44 employed in cellular base stations typically use copper power supply and return conductors (or alloys thereof) that have physical properties which are familiar to those skilled in the art. One important property of these conductors is their electrical resistance. The electrical resistance of a conductor of the power cable 36 (or breakout cord 44) is inversely proportional to the diameter of the conductor (assuming a conductor having a circular cross-section). Thus, the larger the diameter of the conductors (i.e., the lower the gauge of the conductor), the lower the resistance of the power cable 36. Copper resistance is specified in terms of unit length, typically milliohms (mΩ)/ft; as such, the cumulative electrical resistance RCable of the power cable 36 and the breakout cord 44 increases with the lengths of the cable 36 and the breakout cord 44. Typically, the breakout cords 44 are much shorter than the power cables 36, and hence the power cable 36 is the primary contributor to the cumulative resistance. Thus, the longer the power cable 36, the higher the voltage drop VDrop. This effect is well understand and is typically accounted for by engineering and the system architects.
Remote radio heads 24 are typically designed to operate properly if supplied with a DC power signal having a voltage within a specified range. Conventionally, the power supply 26 at the base of the tower 30 will be set to output a DC power signal having a fixed voltage VPS. As VDrop is a function of the current IRRH that is supplied to the remote radio head 24 (see Equation 2 above), the voltage VRRH of the power signal that is delivered to the remote radio head 24 will change with variation in the current IRRH drawn by the remote radio head 24 due to variation in the voltage drop VDrop. If VDrop becomes too large, then the voltage of the power signal that is supplied to the remote radio head 24 may fall below the minimum voltage that is necessary to properly power the remote radio head 24.